The invention relates to a brake system as defined hereinafter. Various embodiments of brake systems of this type, equipped with a traction control system, are known and are in use. Typically, the master brake cylinder piston is decoupled from the brake pedal. Such brake systems are proposed for instance in U.S. Pat. Nos. 4,620,750 and 4,660,897. In these systems, so-called 3/3-way monitor control valves monitor the brake booster function upon feeding of pressure, brake pressure failure, and energy failure. As a result, the effect attained is that the brake circuit is subjected to pressure, regardless of the actuation of the brake pedal. The action thus attained is used particularly in traction control.